The present invention relates generally to a power transfer system for controlling the distribution of drive torque between the primary and secondary drivelines of a four-wheel drive vehicle. More particularly, the present invention relates to an on-demand transfer case having a two-speed gear reduction unit, a range shift mechanism for establishing high-range and low-range drive modes, and a controllable bi-directional overrunning mode clutch for controlling torque transfer from the primary driveline to the secondary driveline.
Four-wheel drive vehicles are in great demand due to the enhanced on and off road traction control they provide. In many four-wheel drive vehicles, a transfer case is installed in the drivetrain and is normally operable to deliver drive torque to the primary driveline for establishing a two-wheel drive mode. The transfer case is further equipped with a clutch assembly that can be selectively or automatically actuated to transfer drive torque to the secondary driveline for establishing a four-wheel drive mode. These xe2x80x9cmodexe2x80x9d clutch assemblies can range from a simple dog clutch that is operable for mechanically shifting between the two-wheel drive mode and a xe2x80x9clockedxe2x80x9d (i.e., part-time) four-wheel drive mode to a more sophisticated automatically-actuated multi-plate clutch for providing an xe2x80x9con-demandxe2x80x9d four-wheel drive mode.
On-demand four-wheel drive systems are able to provide enhanced traction and stability control and improved operator convenience since the drive torque is transferred to the secondary driveline automatically in response to lost traction at the primary driveline. An example of passively-controlled on-demand transfer case is shown in U.S. Pat. No. 5,704,863 where the amount of drive torque transferred through a pump-actuated clutch pack is regulated as a function of the interaxle speed differential. In contrast, actively-controlled on-demand transfer cases include a clutch actuator that is adaptively controlled by an electronic control unit in response to instantaneous vehicular operating characteristics detected by a plurality of vehicle sensors. U.S. Pat. Nos. 4,874,056, 5,363,938 and 5,407,024 disclose various examples of adaptive on-demand four-wheel drive systems.
Due to the cost and complexity associated with actively-controlled clutch assemblies, recent efforts have been directed to the use of overrunning clutches that can be easily controlled to provide various operating modes. For example, U.S. Pat. No. 5,993,592 illustrates a pawl-type controllable overrunning clutch assembly installed in a transfer case and which can be shifted between various drive modes. U.S. Pat. No. 6,092,635 discloses a hydraulically-actuated multi-function controllable overrunning clutch assembly that is noted to be operable for use in vehicular power transmission mechanisms. Likewise, U.S. Pat. Nos. 5,924,510, 5,951,428, 6,123,183, and 6,132,332 each disclose a controllable multi-mode overrunning clutch installed in a transfer case and which is actuated using an electromagnetic clutch. In view of this recent interest, a need exists to continue development of controllable bi-directional overrunning clutch assemblies which provide improved structure, robust operation, and reduced packaging for use in four-wheel drive transfer cases.
It is therefore an object of the present invention to provide a transfer case for an on-demand four-wheel drive vehicle having a two-speed gear reduction unit and a range shift mechanism which can be selectively actuated for establishing various four-wheel high-range and low-range drive mode.
According to another object of the present invention, the two-speed transfer case includes a controllable multi-mode bi-directional overrunning clutch assembly which is for controlling the transfer of drive torque to the secondary driveline in response to the occurrence of traction loss at the wheels of the primary driveline.
It is a further object of the present invention to provide a power-operated actuator to control shifting of the bi-directional overrunning clutch assembly between its distinct modes in response to mode signals received by a controller.
The present invention is directed to integration of a controllable, multi-mode, bi-directional overrunning clutch assembly and a mode shift system into a four-wheel drive transfer case for limiting slip between a primary output shaft and a secondary output shaft. The clutch assembly includes a first ring journalled on a first rotary member, a second ring fixed to a second rotary member, and a plurality of rollers disposed in opposed cam tracks formed between the first and second rings. The first ring is split to define an actuation channel having a pair of spaced end segments. An actuator ring is moveable between positions engaged with and released from the end segments of the first ring. The mode shift system is operable to control movement of the actuator ring for establishing an automatic four-wheel drive mode and a locked four-wheel drive mode.
In accordance with one particular embodiment of the bi-directional overrunning clutch assembly of the present invention, the first ring is journalled on a sprocket driven by the secondary output shaft and the second ring is driven by the primary output shaft. Thus, the invention provides for installing the controllable, multi-mode, bi-directional overrunning clutch assembly in association with the sprocket of a transfer assembly to permit significant axial length reductions for the transfer case.